1. Field of the Invention
The present invention relates to method and apparatus for seismic exploration, and, more particularly, to providing a nonlinear sweep signal to a vibrator in a seismic system.
2. Description of the Prior Art
In seismic exploration, seismic waves are commonly used to probe the earth's crust as a means of determining the type and location of subsurface formations. The earth's crust can be considered a transmission medium or filter whose characteristics are to be determined by passing seismic waves through that medium. In the reflection seismic method, seismic waves or impulses are generated at a point at or near the earth's surface, and the compressional mode of these waves is reflected from subsurface acoustic impedance boundaries and detected by arrays of seismic detectors located at the earth's crust. The seismic detectors convert the received waves into electrical signals which are sensed and recorded in a form which permits analysis. Skilled interpreters can discern from such an analysis the shape and depth of subsurface reflection boundaries and the likelihood of finding an accumulation of minerals, such as oil and gas.
Various sources of seismic energy have been utilized in the art to impart the seismic waves into the earth's crust. Such sources have included dynamite and weight-drop apparatus.
Another source of seismic energy is a vibrator which, when energized, imparts relatively low level energy signals into the earth's crust. Typically, the impartation of energy with vibrator devices is for a preselected energization interval, and data are recorded during the energization interval and a subsequent "listening" interval.
Often it is desirable for the vibrator to impart energies of varying frequencies into the earth's crust during the energization interval. In such instances, energy at a starting frequency is first imparted into the earth, and the frequency of energization changes over the energization interval at some rate until the stopping frequency is reached at the end of the interval. The difference between the starting and stopping frequencies of the sweep generator is known as the range of the sweep, and the length of time in which the generator has to sweep through those frequencies is known as the sweep time.
Vibrators typically employ a sweep generator, and the output of the sweep generator is coupled to the input of the vibrating type device. The output of the sweep generator dictates the manner in which the frequency of the energization signal which is imparted into the earth's crust varies as a function of time.
Several methods of effecting the rate of change of the frequency of the sweep generator during the sweep time have been proposed. For example, in the case of a linear sweep, the frequency output of the sweep generator changes linearly over the sweep time at the rate dictated by the starting and stopping frequencies and the sweep time. Further, nonlinear sweeps have been proposed in which the rate of change of the frequency of the sweep generator varies nonlinearly between the starting and stopping frequencies over the sweep time. Examples of nonlinear sweeps have been quadratic sweeps and square root sweeps.
It is known in the seismic exploration art that the higher frequencies of energization signals are attenuated to a greater degree than lower frequency energization signals, and most authorities have concluded that the attenuation of the earth in decibels is directly proportional to the frequency of the energization signal. Further, the total attenuation of any specific signal is known to be dependent upon the velocity, layering, thickness and attenuation coefficients of each layer traversed, as well as the frequency range.
Even though the earth attenuation is known to increase with increasing frequency of the energization signals, linear sweeps have been extensively used in vibrators. Techniques for emphasizing the lower amplitude higher frequency responses are well-known and have been employed to account for the attenuation applied to these higher frequency seismic signals by the earth.
Nonlinear sweep signals have been suggested but have not achieved acceptance by the industry due to poor performance.
The shortcomings of the prior art are overcome with the method and apparatus of the present invention.